It has been demonstrated that CD8 positive CTLs recognize epitope peptides derived from the tumor-associated antigens (TAAs) found on major histocompatibility complex (MHC) class I molecules, and then kill the tumor cells. Since the discovery of the melanoma antigen (MAGE) family as the first example of TAAs, many other TAAs have been discovered, primarily through immunological approaches (Boon T, Int J Cancer 1993 May 8, 54(2): 177-80; Boon T & van der Bruggen P, J Exp Med 1996 Mar. 1, 183(3): 725-9). Some of these TAAs are currently undergoing of clinical development as immunotherapeutic targets.
Identification of new TAAs, capable of inducing potent and specific anti-tumor immune responses, warrants further development and clinical application of peptide vaccination strategies for various types of cancer (Harris C C, J Natl Cancer Inst 1996 Oct. 16, 88(20): 1442-55; Butterfield L H et al., Cancer Res 1999 Jul. 1, 59(13): 3134-42; Vissers J L et al., Cancer Res 1999 Nov. 1, 59(21): 5554-9; van der Burg S H et al., J Immunol 1996 May 1, 156(9): 3308-14; Tanaka F et al., Cancer Res 1997 Oct. 15, 57(20): 4465-8; Fujie T et al., Int J Cancer 1999 Jan. 18, 80(2): 169-72; Kikuchi M et al., Int J Cancer 1999 May 5, 81(3): 459-66; Oiso M et al., Int J Cancer 1999 May 5, 81(3): 387-94). To date, there have been several reports of clinical trials using these tumor-associated antigen derived peptides. Unfortunately, only a low objective response rate has been observed in these cancer vaccine trials so far (Belli F et al., J Clin Oncol 2002 Oct. 15, 20(20): 4169-80; Coulie P G et al., Immunol Rev 2002 October, 188: 33-42; Rosenberg S A et al., Nat Med 2004 September, 10(9): 909-15).
Recently, algorithms for predicting HLA class I-binding peptide sequences have been developed (Journal of Immunological Methods, (1995), Vol. 185, pp. 181-190, J. Immunol., (1994), Vol. 152, pp. 163-175, protein science, (2000), Vol. 9, pp. 1838-1846). However, it is difficult to estimate if a predicted epitope peptide can be processed naturally in the target cells and expressed on the target cell surface with HLA molecule. Moreover, the algorithms, for example BIMAS (world wide web-bimas.dcrt.nih.gov/cgi-bin/molbio/ken_parker_comboform) (Parker K C, et al., (1994) J Immunol.; 152(1):163-75.; Kuzushima K, et al., (2001) Blood.; 98(6):1872-81.)) can suggest less than rigorous HLA molecule-binding peptides (Bachinsky M M, et. al., Cancer Immun. 2005 Mar. 22; 5:6.). Thus, identifying epitope peptides remains challenging and difficult.
MELK, maternal embryonic leucine zipper kinase, was previously identified as a new member of the snf1/AMPK serine-threonine kinase family that is involved in mammalian embryonic development (Heyer B S et al., Dev Dyn. 1999 August 215(4):344-51). The gene was shown to play an important role in stem cell renewal (Nakano I et al., J Cell Biol. 2005 Aug. 1, 170(3):413-27), cell-cycle progression (Blot J et al., Dev Biol. 2002 Jan. 15, 241(2):327-38; Seong H A et al., Biochem J. 2002 Feb. 1, 361(Pt 3):597-604) and pre-mRNA splicing (Vulsteke V et al., J Biol Chem. 2004 Mar. 5, 279(10):8642-7. Epub 2003 Dec. 29). In addition, through gene expression profile analysis using a genome-wide cDNA microarray containing 23,040 genes, MELK was recently shown to be up-regulated in breast cancer (Lin M L et al., Breast Cancer Res. 2007; 9 (1):R17, WO2006/016525, WO2008/023841). In fact, MELK is up-regulated in several cancer cells, for example lung, bladder, lymphoma and cervical cancer cells (See WO2004/031413, WO2007/013665, and WO2006/085684, the disclosures of which are incorporated by reference herein). Northern blot analysis on multiple human tissues and cancer cell lines demonstrated that MELK was over-expressed at a significantly high level in a great majority of breast cancers and cell lines, but was not expressed in normal vital organs (heart, liver, lung and kidney) (WO2006/016525). Furthermore, suppression of MELK expression by siRNA was shown to significantly inhibit growth of human breast cancer cells. Accordingly, MELK is considered to be a suitable target for cancer immunotherapy and epitope peptides derived therefrom may be expected to serve as cancer immunotherapeutics effective in the treatment of a wide array of cancer types.